Ink-jet printing generally involves the controlled delivery of ink drops from an ink-jet pen reservoir to a printing surface. One type of ink-jet printing, known as drop-on-demand printing, employs a pen that has a print head that is responsive to control signals for ejecting drops of ink from the ink reservoir.
Drop-on-demand type print heads typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave. A thermal bubble type print head includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice.
Piezoelectric pressure wave type print heads use a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of ink in the print head to thereby produce a pressure wave that forces the ink drops through the orifice.
Although conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, they do not include any mechanism for preventing ink from permeating through the print head when the print head is inactive. Accordingly, drop-on-demand techniques require that the fluid in the ink reservoir must be stored in a manner that provides a slight back pressure at the print head to prevent ink leakage from the pen whenever the print head is inactive. As used herein, the term "back pressure" means the partial vacuum within the pen reservoir that resists the flow of ink through the print head. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in the partial vacuum. Accordingly, back pressure is measured in positive terms, such as water column height.
The back pressure at the print head must be at all times strong enough for preventing ink leakage. The back pressure, however, must not be so strong that the print head is unable to overcome the back pressure to eject ink drops. Moreover, the ink-jet pen must be designed to operate despite environmental changes that cause fluctuations in the back pressure.
A severe environmental change that affects reservoir back pressure occurs during air transport of an ink-jet pen. In this instance, ambient air pressure decreases as the aircraft gains altitude and is depressurized. As ambient air pressure decreases, a correspondingly greater amount of back pressure is needed to keep ink from leaking through the print head. Accordingly, the level of back pressure within the pen must be regulated during times of ambient pressure drop.
The back pressure within an ink-jet pen reservoir is subjected to what may be termed "operational effects". One significant operational effect occurs as the print head is activated to eject ink drops. The consequent depletion of ink from the reservoir increases (makes more negative) the reservoir back pressure. Without regulation of this back pressure increase, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased back pressure to eject ink drops.
Past efforts to regulate ink-jet reservoir back pressure in response to environmental changes and operational effects have included mechanisms that may be collectively referred to as accumulators.
Generally, prior accumulators comprise an elastomeric bladder or cup-like mechanism that defines a volume that is in fluid communication with the ink-jet pen reservoir volume. The accumulators are designed to move between a minimum volume position and a maximum volume position in response to changes in the level of the back pressure within the reservoir. Accumulator movement changes the overall volume of the reservoir to regulate back pressure level changes so that the back pressure remains within an operating range that is suitable for preventing ink leakage while permitting the print head to continue ejecting ink drops.
For example, as the difference between ambient pressure and the back pressure within the pen decreases as a result of ambient air pressure drop, the accumulator moves to increase the reservoir volume to thereby increase the back pressure to a level, within the range discussed above, that prevents ink leakage. Put another way, the increased volume attributable to accumulator movement prevents a decrease in the difference between ambient air pressure and back pressure that would otherwise occur if the reservoir were constrained to a fixed volume as ambient air pressure decreased.
Accumulators also move to decrease the reservoir volume whenever environmental changes or operational effects (for example, ink depletion occurring during operation of the pen) cause an increase in the back pressure. The decreased volume attributable to accumulator movement reduces the back pressure to a level within the operating range, thereby permitting the print head to continue ejecting ink.
Accumulators are usually equipped with internal or external resilient mechanisms that continuously urge the accumulators toward a position for increasing the volume of the reservoir. The effect of the resilient mechanisms is to retain a sufficient minimum back pressure within the reservoir (to prevent ink leakage) even as the accumulator moves to increase or decrease the reservoir volume.
Prior accumulator designs suffer from at least two deficiencies. First, the working volume of the accumulator (that is, the maximum reservoir volume increase or decrease that is provided by the accumulator) was limited in size. Specifically, the working volume of the accumulator was limited to the maximum size of the bladder or similar structure that could be housed within the ink-jet pen. Accordingly, the environmental operating range of prior pens, which range may be quantified as the maximum ambient pressure drop the pen could sustain without leakage, was limited by the size of the working volume of the accumulator.
One prior approach to overcoming the working volume size limitation just described lead to the inclusion of a catch basin within the ink-jet pen. The catch basin provides a volume for receiving through an overflow orifice ink that is forced out of the reservoir as ambient pressure continues to drop after the accumulator moves into its maximum reservoir volume position. The continued drop in ambient pressure eventually eliminates the difference between ambient pressure and the back pressure within the reservoir. Eventually, a low-level positive pressure develops within the reservoir. The low-level positive pressure forces the ink through the overflow orifice into the catch basin. The inclusion of the overflow orifice and catch basin is intended to prevent the positive pressure in the reservoir from rising to a level that would permit ink to leak out of the inactive print head.
Use of catch basins is undesirable because they employ space within the ink-jet pen assembly that could otherwise be used as ink reservoir space. Moreover, it is difficult to design the pen so that ink is forced through the overflow orifice but not through the print head.
A second deficiency in prior accumulator designs pertains to a feature known as drawdown. Drawdown is the amount of ink volume that must be withdrawn from a filled ink-jet pen in order to establish within the reservoir a minimum back pressure to ensure ink does not leak through the print head. This minimum back pressure is typically established at the time the pen is filled with ink, that is, at the time the air volume in the reservoir is minimal. It is desirable to remove as little "drawdown" ink as possible in order to establish the minimum back pressure since the withdrawal of ink for this purpose reduces the amount of ink that can be used for printing.
Prior accumulators, being formed of moldable elastomers, generally allow significant volumes of air to diffuse through their walls. Correspondingly, larger drawdown volumes were required in prior accumulators so that the addition of air into the reservoir by diffusion did not cause the accumulators to expand to their maximum volume. It can be appreciated that the reservoir back pressure is lost when the accumulators attain their maximum volume.